drm/amd/display: fix FCLK pstate change underflow

drm/amd/display: fix FCLK pstate change underflow

[ Upstream commit 481a116ba75f936e7d30f142d90b15e15009f74e ]

[Why]
Currently we set FCLK p-state change
watermark calculated based on dummy
p-state latency when UCLK p-state is
not supported

[How]
Calculate FCLK p-state change watermark
based on on FCLK pstate change latency
in case UCLK p-state is not supported

Reviewed-by: Nevenko Stupar <Nevenko.Stupar@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Vladimir Stempen <vladimir.stempen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>